Expansion of the market for displays in recent years has been accompanied by increased demand for viewing of vivid images, and thus there is a need for optical materials provided with higher level optical properties in addition to transparency, heat resistance, and strength.
Acrylic resins ((meth)acrylic acid ester polymers) are attracting attention for use as such optical materials from viewpoints of transparency, surface hardness, optical properties, and so forth.
It has previously been reported that among acrylic resins, those having enhanced heat resistance through copolymerization of a (meth)acrylic acid ester monomer with glutaric anhydride (for example, refer to PTL 1), maleic anhydride (for example, refer to PTL 2), or the like are particularly good as optical materials.
However, a disadvantage of acrylic resins that have enhanced heat resistance as described above (heat resistant acrylic resins) is that they are susceptible to thermal decomposition in high-temperature shaping compared to a general-purpose acrylic resin (i.e., a copolymer of methyl methacrylate and an acrylic acid ester).
Moreover, since progress toward larger and thinner (for example, film form) shaped products has been accompanied by shaping at high temperature and longer holding time at high temperature, another disadvantage of heat resistant acrylic resins is that foaming may occur during shaping processing.
Furthermore, heat resistant acrylic resins have weak strength and low toughness, which leads to a problem of poor producibility in terms of film shaping processability and handleability.
In terms of conventional techniques for improving the strength of a heat resistant acrylic resin, a technique of including a crosslinked elastic body having a multilayer structure in an acrylic resin including a glutaric anhydride-based unit has been disclosed (for example, refer to PTL 3).
A technique of adding acrylic rubber to an acrylic resin including a maleic anhydride unit has also been disclosed (for example, refer to PTL 4).
Moreover, a technique of adding a multilayer structure rubber and a thermal stabilizer to an acrylic resin including a maleic anhydride unit (for example, refer to PTL 5), and a technique of including a rubbery polymer and, as necessary, an ultraviolet absorber in an acrylic resin including a structural unit having a cyclic structure-containing main chain (for example, refer to PTL 6) have been disclosed.
Furthermore, a technique for reducing the content of residual monomer in a resin has been disclosed (for example, refer to PTL 7).